Use of a nucleic acid/pei complex

ABSTRACT

The invention concerns the use of a nucleic acid/cationic polymer complex, preferably polyethyleneimine (PEI for preparing a composition for intraventricular stereotactic screening of stem cells of the brain for preparing a medicine for treating neurodegenerative and/or demyelinating disease. The invention further concerns a method for obtaining an animal whereof the genome of stem cells of the brain are modified by using said complex. The invention also concerns a method for obtaining an animal for screening compounds designed to modify the disposition of stem cells of the brain.

The present invention relates to biology, and more particularly to gene therapy. The present invention relates to a novel use of a nucleic acid/cationic polymer, and in particular nucleic acid/polyethyleneimine, complex for preparing a composition intended for the targeting of stem cells of the brain. Such a composition is particularly intended for the preparation of a medicinal product intended for the treatment of neurodegenerative and/or demyelinating diseases. The invention also relates to a method of producing an animal, with the exception of humans, in which at least one stem cell of the brain has undergone an event of site-specific recombination targeted to a DNA sequence of interest. The invention also relates to the animal which can be obtained using the method.

Neurodegenerative diseases constitute a major and increasing problem for public health among the aging population. For this reason, a great deal of effort is currently directed toward the development of protocols for transferring genes into the brain. Such protocols are difficult to develop because they require, inter alia, having effective and safe transfer vectors, and they require the difficulties specific to delivering DNA into the brain to be overcome, namely the existence of the blood-brain barrier, which aims to prevent vascular delivery of the vectors, and the post-mitotic state of the neuronal population.

With regard to transfer vectors, significant success in delivering DNA into the mature brain has been obtained with experimental approaches using direct injection of viral vectors based on DNA viruses such as adenoviruses (Akli et al., 1994; Bajocchi et al., 1993; Davidson et al., 1993; Le Gal La Salle, et al., 1993), the vectors derived from the herpes virus (Boviatsis et al., 1994; Pakzaban et al., 1994; Wood et al., 1994) and adenovirus-associated viruses (AAV) (Kaplitt et al., 1994). More recently, a retroviral system based on the human immunodeficiency virus (HIV) has been used to mediate the stable transduction of neurons in the rat brain (Naldini et al., 1996).

Although efforts are mainly directed toward viral vectors, nonviral techniques constitute varied alternatives full of potential. Specifically, chemical transporters of DNA have many advantages over viral vectors. Their use provides greater flexibility, and they are simpler to prepare, to purify and to store. Safety data and toxicity data are also easier to obtain. They can be used with any sort of DNA, thus allowing the use of long nucleic acid sequences, such as a whole gene, associated with plasmidic vectors.

Among the nonviral vectors, two main classes of molecules can be distinguished overall: cation lipids, such as DOGS (diocade cylamidoglycyl spermine) or Transfectam® (Behr et al., 1989) or Lipofectin® (Felgner et al., 1987), and DNA-binding polymeric cations, such as poly-L-Lysine (PLL), protamine, “cationized” albumin, polyethyleneimine (PEI) (Boussif et al., 1995), block copolymers (Read et al., 2000; Oupicky et al., 2000; Wolfert et al., 1999), and polyamidoamine dendrimers (Tang et al., 1996; Planck et al., 1999).

The use of cationic polymers, and in particular of PEI, for delivering genes in vivo has shown that these vectors can provide high levels of expression in the brain (Abdallah et al., 1996; Boussif et al., 1995; Goula et al., 1998; Lemkine et al., 1999).

In order to bypass the difficulty constituted by the “blood-brain” barrier, other routes of administration have been developed, such as direct injection of virus, such as the attenuated herpes simplex virus (Chambers et al., 1995) or sterotactic injection, such as injection of DNA/PEI complex (Abdallah el al., 1996). Another possible route of administration consists of the local production of therapeutic peptide or protein; thus, cell lines transformed with retroviruses have been used to produce therapeutic proteins in order to correct lysomsomal disease affecting the brain (Snyder et al., 1995) and brain tumors have been treated with cells producing retroviruses (Culver et al., 1992; Barba et al., 1994).

The third major obstacle to the delivery of DNA in the CNS, consisting of the post-mitotic state of the vast majority of neuronal cells, represents, to date, an obstacle which is difficult to overcome.

The present invention therefore proposes to solve this problem by specifically targeting the cells of the central nervous system (CNS) which have conserved their ability to divide and to differentiate. Specifically, cells which correspond to neuronal stem cells have recently been demonstrated in vivo using retroviruses, or using a marker such as thymidine or bromodesoxyuridine (BrdU); since retroviruses integrate only in dividing cells, they make it possible to follow the entire cell descendance, thus reflecting a particular lineage. After several years of studies, two sites have now been recognized as being the main sites of cell proliferation in the adult brain: the subventricular zone and the subgranular zone of the hippocampus dentate gyrus. However, not all the cells of these regions have the proliferative potential of the particular cell subpopulation constituted by the stem cells. The exact phenotype of the most primitive cell in these regions is still poorly understood, but recent articles have shed light on this complex question. Johansson et al., (1999) have provided a demonstration that a subpopulation of ependymal cells bordering the third ventricle are stem cells.

Subsequently, Doetsch et al., (1999) have presented clearer arguments in favor of the idea that the progenitor cells belong to a class of subventricle cells which express the GFAP (Glial Fibrilly Acidic Protein) marker, which indicates that the stem cells would be subependymal astrocytes. Moreover, a third group has shown that, even if these two types of cell were capable of division, only the subependymal astrocytes could renew themselves and give rise to neurons and glial cells (Chiasson et al., 1999). Another marker makes it possible to characterize the progenitor cells of the central nervous system: the protein nestin, which constitutes an intermediate filament of the cytoskeleton (McKay et al., 1997).

It therefore appears to be of major interest to develop a protocol for specifically targeting a transgene into the embryonic stem cells of the adult brain. This is the problem that the present invention proposes to solve. The studies of the prior art, and in particular those of Goula et al., (1998), have not made it possible to achieve this aim. Specifically, the article by Goula et al., (1998), which reports preliminary studies regarding the setting up of the use of PEI in the central nervous system in a general way (i.e. both in the newborn and in the adult) describes a transfection procedure which does not make it possible to specifically target the adult neural cell stem population; the article by Goula et al., (1998) shows that the various main cell. types of the brain, i.e. the glial cells and the neurons, are indifferently transfected and express the transgene. Entirely fortuitously, the inventors have solved. the problem posed by using a nucleic acid/cationic polymer complex to prepare a composition intended for the targeting of stem cells of the adult brain.

The solving of the technical problem has been obtained by introducing, preferably by injecting, the composition according to the invention into the adult brain, and preferably into one or more ventricles of the brain, close to or into the ventricular stem cells of the brain, preferably of the adult brain. The composition according to the invention is preferably injected into the brain ventricle(s) where the embryonic stem cells of the brain (i.e. the ventricular stem cells of the brain) have been located. Even more preferably, this involves one or more lateral ventricles. In addition, the composition according to the invention can be injected into the 3rd or 4th ventricle and then diffused into the cephalic fluid so as to reach the ventricular embryonic stem cells. The composition according to the invention is introduced, preferably by intraventricular injection into the adult brain, the injection of said composition being carried out stereotactically and said injection lasting at least 10 minutes, optionally at least 15 minutes, or at least 20 minutes.

The solving of the technical problem has also been obtained by using small amounts of nucleic acids and small injection volumes. Specifically, to target the stem cells of the adult mouse brain, the amount of said nucleic acid present in said composition should be at least less than 2.5 μg. Thus, it may be less than 2 μg, than 1.75 μg, than 1.5 μg, than 0.75 μg, than 0.5 μg, than 2.5 μg, than 0.1 μg, than 0.05 μg, or than 0.01 μg. In addition, for the targeting of the stem cells of the adult mouse brain, too large a volume of said composition should not be injected, and it should not under any circumstances exceed 5 μl, since larger volumes lead to a reflux at the time of injection and are liable to cause damage to the tissues. Thus, the volume of composition injected into the adult mouse brain is less than 5 μl, optionally less than 4 μl, less than 3 μl, less than 2 μl or less than 1 μl.

These various volumes, injection time and amount of nucleic acid present in the composition according to the invention should be adjusted by those skilled in the art when the injection is carried out in an animal other than the mouse. This adjustment is within the scope of those skilled in the art who, without any excessive effort, can rapidly determine the parameters for use of the composition according to the invention for targeting the ventricular embryonic stem cells of an adult brain. They can thus, before proceeding experimentally, determine the volume of the cerebral ventricles and in particular of the lateral ventricles in the mouse and in the target animal (by magnetic resonance imaging for example) and, by a simple rule of three, determine the amount of nucleic acids, the volume and also the injection time necessary to reproduce the results obtained in the mouse. For this reason, the present invention relates to the use of a nucleic acid/cationic polymer complex, for preparing a composition intended for the targeting of ventricular stem cells of the adult brain of an animal, characterized in that the amount of said nucleic acid present in said composition is adjusted as a function of the volume of the cerebral ventricle of said animal and is determined proportionally to the amount used to target the stem cells of the adult mouse brain, the concentration of said nucleic acid present in the ventricle of said animal being less than or equal to that used for the targeting of the stem cells of the adult mouse brain. The present invention also relates to the use of a nucleic acid/cationic polymer complex, for preparing a composition intended for the targeting of ventricular stem cells of the adult brain, characterized in that the volume of said composition is adjusted as a function of the volume of the cerebral ventricle of said animal and is determined proportionally to the volume used to target the stem cells of the adult mouse brain.

The cell targeted with the compound of the present invention is a eukaryotic cell of an animal. Preferably, the animal according to the invention is a vertebrate, more preferably a mammal, preferably chosen from the group composed of mice, rats, rabbits, hamsters, guinea pigs, bovines, members of the goat family, members of the sheep family, horses and primates, including humans. The animal is preferably a human, the stem cell from the brain being a human cell. According to another preferred embodiment, the animal is a mouse and the stem cell is a murine cell.

The examples and results below demonstrate the favored transfection of adult neural stem cells in particular regions of the brain.

The present invention is not limited only to the targeting of stem cells of the brain, but also to cells of the brain which are not in the post-mitotic state and which are capable of dividing; in this respect, mention may be made of the cells of brain tumors, such as, for example, gliomas or astrocytomas.

According to a preferred embodiment of the invention, said composition is introduced stereotactically. However, any means for delivering the composition of the invention close to or into the stem cells of the brain can be envisioned; in this respect, mention should be made of all the systems for targeting across the blood/brain barrier systemically.

According to a preferred embodiment, said cationic polymer is chosen from the group composed of the polycationic polymers such as polyethyleneimine (PEI), poly-L-lysine, poly-D-lysine, polyamidoamine, polyamine, block copolymers (Read et al., 2000; Oupicky et al., 2000; Wolfert et al., 1999), and polyamidoamine dendrimers (Tang et al., 1996; Plank et al., 1999). Preferably, the polycationic polymer is polyethyleneimine (PEI). Preferably it is low molecular weight PEI, more preferably it is PEI having an average molecular weight of less than or equal to 88 kDa, less than or equal to 44 kDa, less than or equal to 35 kDa, less than or equal to 30 kDa, less than or equal to 22 kDa, or less than or equal to 11 kDa. Even more preferably, the PEI used is a PEI of 22 kDa (EXGENE 500®, EUROMETEX Soufflemeyersheim, France). Other cationic polymers can optionally be used, such as nucleic acid binding proteins, among which mention should be made of histones, protamine, ornithine, putrescine, spermidine and spermine. In addition, the possibility exists of grafting the cationic polymer with proteins, antibodies or membrane receptor ligands, making it possible to specifically target cells by facilitating internalization of the complexes.

The nucleic acid/cationic polymer complex according to the invention is intended for the preparation of a medicinal product intended for the treatment of neurodegenerative and/or demyelinating diseases. Among these diseases, mention should be made, in a nonexhaustive manner, of diseases such as Alzheimer's disease, Parkinson's disease, Huntington's chorea and multiple sclerosis. The complex according to the invention can also be used to prepare a medicinal product intended to modify the evolution of the stem cells of the brain and/or to increase the survival of the stem cells of the brain.

For the preparation of these medicinal products, said nucleic acid is chosen from single-stranded DNA, double-stranded DNA, single-stranded RNA, double-stranded RNA and RNA/DNA hybrid.

According to a preferred embodiment, said nucleic acid is double-stranded DNA or single-stranded RNA which at least encodes a protein product of interest which is expressed effectively in said stem cell of the brain.

Said protein product of interest is chosen from the group composed of pro-apoptotic or anti-apoptotic proteins, survival factors, differentiation factors, cytokines, lymphokines, interleukins, growth factors, transcription factors, killer proteins, recombinases, integrases, transposases, enzymes involved with nucleic acids, and “reporter” proteins.

Among the pro- or anti-apototic proteins which may or may not be involved in the mitochondrial pathway, mention should be made, in a nonexhaustive manner, besides the proteins of the Bc12 family (such as BClX_(L), BCLX_(s), Bcl_(w), Bid, Bax, Bak), the proteins IAP, SMAC and Diablo. Preferably, said protein product of interest is the BcLX_(L) protein.

The interleukines, cytokines and lymphopkines are chosen from a group preferably composed of interleukines I1-1, I1-2, I1-3, I1-4, I1-5, I1-6, I1-7, I1-8, I1-0, I1-10, I1-11, I1-12, I1-13, I1-14, I1-15, I1-16, I1-17 and I1-18, and interferons α-IFN, β-IFN and γ-IFN.

The growth factors are preferably epithelial growth factor (EGF), fibroblast growth factor (FGF, and bFGF, basic fibroblast growth factor), platelet-derived growth factor (PGDF), nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and glial-derived growth factor (GDGF); mention should also be made of the colony stimulating factors, such as G-CSF, GM-CSF, or M-CSF, and erythropoietine. Mention should also be made of the growth factors which interact, by inhibiting, with nucleic transcription factors such as NF-Kβ.

Among the transcription factors, mention should be made of the transcription factors which make it possible to direct the expression of a gene in a particular cell type or at a given moment of brain cell differentiation, and mention should be made of the transcription factors involved in the differentiation of neuronal cells, such as the neurogenins, and glial cells, such as GCM (glial cell missing). Among these factors, mention should be made, in a nonexhaustive manner, of the Dlx, Otx, Emx and Hox families.

It may also be an advantage to target the expression of killer proteins in the stem cells of the brain, or in cells of the brain in an active division phase. In this case, it is advantageous to use a nucleic acid molecule which encodes a protein product of interest chosen from killer proteins; among the killer proteins, mention should be made of kinases, and preferably thymidine kinase, and the pro-apoptotic proteins; the term “pro-apoptotic proteins” is intended to denote the proteins which are involved in apoptose or promote apotose. Among the pro-apoptotic proteins, mention should be made of the BIK (Bcl2-interacting protein), BAX (Oltvai et al., 1993), BAK (Chittenden et al., 1995; Kiefer el al., 1995) and BID (BH3-interacting domain death agonist) (Wang et al., 1996) proteins. Among the pro-apoptotic proteins, mention should also be made of caspases, the AIF (apoptosis-inducing factor) protein (Susin el al., 1999) and the proteins of the tumor necrosis factor (TNF) family, and more particularly TNF itself (Old 1985) and the FASL (FAS-ligand) protein (Takahashi et al., 1994).

The term “recombinase protein” is intended to denote the recombinases of the integrases family which catalyze the excision, insertion, inversion or translocation of DNA fragments at specific sites of recognition for said recombinases (Sternberg et al., 1986; Sauer, et al., 1990; Barbonis et al., 1993; Kilby et al., 1993, Sauer, 1994, Denisen et al., 1995). These recombinases are active in animal cells (Sauer, 1994). The recombinase protein of the invention is preferably selected from the group of site-specific recombinases composed of the bacteriophage P1 Cre reconxiinase, the Saccharomyces cerevisiae FLP recombinase, the Zygosaccharomyces rouxii pSRI recombinase R, the Kluyveromyces drosophilarium pKD1 recombinase A, the Kluyveromyces waltii pKWl recombinase A, the λ Int integrase, and the recombinase of the Mu phage GIN recombination system, or a variant thereof.

According to a preferred embodiment, the recombinase is the Cre (cyclization recombination) recombinase, which is a 38 KDa integrase of the bacteriophage P1 which catalyzes, in the absence of cofactors, recombination between two DNA sequences of 34 base pairs called “loxP site” (Sauer et al., 1990). The position on one or more DNA molecules and the orientation of loxP sites compared to one another determine the type of function of the Cre recombinase: an excision, insertion, inversion or translocation. Thus, the recombinase activity of Cre is an inversion when two loxP sites are head-to-tail on the same DNA fragment, and an excision when the loxP sites are in direct repetition on the same DNA fragment. The activity of the recombinase is an insertion when a loxP site is present on a DNA fragment, it being possible for a DNA molecule such as a plasmid containing a loxP site to be inserted at said loxP site. The Cre recombinase can also induce a translocation between two chromosomes, on condition that a loxP site is present on each one of them (Babinet, 1995). More generally, the Cre recombinase is therefore capable of catalyzing recombination between one or more different DNA molecules, on condition that they carry loxP sites. One of the objects of the present invention is therefore to use a nucleic acid/PEI complex for the targeting of stem cells, so as to introduce therein a vector expressing the site-specific recombinase, in which the site-specific

Among the transposases, mention should be made of the patented “transposon sleeping beauty” system (Kay et al., 2000) or any other system for introducing a plasmid with IR (“inverted repeat”) sequences framing a nucleic acid sequence of interest, with another expression vector plasmid for a transposase specific for the IR sequences, by cotransfection.

Such transposases catalyze the transposition and/or the integration of a DNA sequence of interest into the genome of said stem cell, it being possible for this DNA sequence of interest to be introduced into said cell using the complex according to the invention.

Among the enzymes involved with nucleic acids, mention should be made of the enzymes involved with DNA, such as restriction enzymes, DNA polymerases or ligases, and the enzymes involved with RNA, such as DNA-dependant RNA polymerases or reverse transcriptases.

According to another embodiment of the invention, the nucleic acid molecule is an antisense RNA, a double-stranded RNA, or a DNA/RNA chimera. Such RNA molecules can be used to inhibit the expression of a protein product of interest.

It is evident that the compound according to the invention has many uses, depending on the nature of the DNA sequence. These many uses can be easily envisioned by those skilled in the art and cannot be mentioned exhaustively. However, it should be underlined that the protein of interest encoded by the nucleic acid, or the antisense RNA, can be used to modify the expression of at least one cellular or mitochondrial gene, i.e. to decrease, to increase, to modulate or to destroy the expression of said gene. The protein of interest can also be used to induce the expression of at least one gene of interest; in this case, the protein of interest is preferably a transcription factor.

According to another embodiment, the protein of interest is a “reporter” protein. Among the “reporter” proteins, mention should be made, in a nonexhaustive manner, of luciferase, green fluorescence protein (GFP), β-galactosidase (β-gal) and chloramphenicol acetyltransferase (CAT). Such “reporter” protein can be used to follow the evolution of the stem cells in the brain. One of the objects of the present invention is therefore to use the complex according to the invention for targeting stem cells of the brain, in which the nucleic acid encodes a “reporter” protein or constitutes a signal-generating marker in order to enable detection, localization and imaging of stem cells of the brain.

The nucleic acid constitutes a signal-generating marker when said nucleic acid is labeled with radioactive isotopes or with non isotopic entities. The non isotopic entities can be selected from enzymes, dyes, haptenes, luminescent agents, such as radio luminescent, chemiluminescent, bioluminescent, fluorescent or phosphorescent agents, and ligands, such as biotin, avidin, streptavidin or digoxigenin. The methods for revealing and detecting these markers are well known to those skilled in the art.

The present invention therefore provides an effective system which allows the active or passive transport of the nucleic acid molecule across the cytoplasmic membrane, transport to the nucleus, entry into the nucleus and maintenance of this molecule in the functional state in the nucleus. The persistence of the expression of the protein product encoded by the DNA molecule is obtained either by stable integration of the DNA molecule into the chromosomal DNA of the target cell, or by maintenance of the DNA molecule in the episomal form. For certain uses, transient expression with a plasmid in the episomal form may be sufficient to obtain the desired results.

A subject of the invention is also a method of producing an animal, with the exception of humans, in which at least one stem cell of the brain has undergone at least one event of site-specific recombination targeted to a DNA sequence of interest, characterized in that said method comprises the steps of:

-   -   (a) obtaining a totipotent embryonic stem (ES) cell modified by         insertion of (a) recognition site(s) for said site-specific         recombinase protein into said DNA sequence(s) of interest,         located in one or more chromosomes, by homologous recombination;     -   (b) introducing said modified totipotent embryonic stem cell         into an embryo of said organism;     -   (c) selecting the individuals having integrated the genetic         modification into the germinal cells;     -   (d) developing said animal;     -   (e) introducing at least one said nucleic acid/cationic polymer         complex into the brain of said animal obtained in step (d) close         to or into the ventricular stem cells, said nucleic acid at         least encoding a recombinase protein capable of catalyzing the         recombination between said recognition sequences for said         recombinase;     -   (f) expressing said recombinase protein in said cell.

According to a particular embodiment, at least a second nucleic acid/cationic polymer complex can be introduced into the brain of said animal in step (e), and said second nucleic acid is a gene of interest as previously 

1-39. (canceled)
 40. A method for targeting stem cells of an adult brain, comprising introducing into said adult brain by intraventricular injection close to or into ventricular stem cells of said adult brain a nucleic acid/cationic polymer complex composition.
 41. The method as claimed in claim 40, wherein said composition is injected stereotactically for at least 10 minutes.
 42. The method as claimed in claim 40, wherein said cationic polymer is polyethyleneimine.
 43. The method as claimed in claim 40, wherein said adult brain is an adult mouse brain, the amount of said nucleic acid present in said composition being less than 2.5 μg.
 44. The method as claimed in claim 43, wherein the volume of said composition does not exceed 5 μl.
 45. The method as claimed in claim 40, wherein said adult brain is an adult animal brain and the amount of said nucleic acid present in said composition is selected as a function of the volume of a cerebral ventricle of said adult animal brain and is determined proportionally to the amount used to target the stem cells of an adult mouse brain, the concentration of said nucleic acid present in the cerebral ventricle of said adult animal brain being less than or equal to that used for the targeting of the stem cells of the adult mouse brain.
 46. The method as claimed in claim 40, wherein said adult brain is an adult animal brain and the volume of said composition is selected as a function of the volume of a cerebral ventricle of said adult animal brain and is determined proportionally to the volume used to target the stem cells of an adult mouse brain.
 47. The method as claimed in claim 45, wherein said adult animal brain is an adult human brain.
 48. The method as claimed in claim 40, wherein said composition is injected for the treatment of a neurodegenerative and/or demyelinating disease.
 49. The method as claimed in claim 48, wherein said disease is Alzheimer's disease, Parkinson's disease, Huntington's chorea or multiple sclerosis.
 50. The method as claimed in claim 40, wherein said composition is injected for modifying the evolution of the stem cells of the brain.
 51. The method as claimed in claim 40, wherein said composition is injected for increasing the survival of the stem cells of the brain.
 52. The method as claimed in claim 40, wherein said nucleic acid is chosen from single-stranded DNA, double-stranded DNA, single-stranded RNA, double-stranded RNA or a RNA/DNA hybrid.
 53. The method as claimed in claim 52, wherein said nucleic acid is double-stranded DNA or single-stranded RNA which encodes at least a protein product which is expressed effectively in said stem cells.
 54. The method as claimed in claim 53, wherein said protein product is a cytokine, lymphokine, interleukine, transcription factor, survival factor, anti-apoptotic protein, pro-apoptotic protein, killer protein, growth factor, recombinase, integrase, transposase, enzyme involved with a nucleic acid, differentiation factor or reporter protein.
 55. The method as claimed in claim 54, wherein said protein product is anti-apoptotic protein BCl-X_(L).
 56. The method as claimed in claim 54, wherein said protein product is a member of a neurogenine family.
 57. The method as claimed in claim 54, wherein said protein product is a site-specific recombinase which is bacteriophage P1 Cre recombinase, Saccharomyces cerevisiae FIP recombinase, Zygosaccharomyces rouxii pSRI recombinase R, Kluyveromyces drosophilarium pKDI recombinase A, Kluyveromyces waltii pKWI recombinase A, λ Int integrase, the recombinase of a Mu phage GIN recombination system, or a variant thereof.
 58. The method as claimed in claim 57, wherein said site-specific recombinase catalyzes the recombination of a fragment of DNA of a genome of said stem cells of the brain.
 59. The method as claimed in claim 54, wherein said protein product is a transposase.
 60. The method as claimed in claim 59, wherein said transposase catalyzes the transposition and the integration of a DNA sequence into a genome of said stem cell of the brain.
 61. The method as claimed in claim 52, wherein said nucleic acid is an antisense RNA.
 62. The method as claimed in claim 59, wherein said composition is injected for modifying the expression of a gene.
 63. The method as claimed in claim 59, wherein said composition is injected for inducing the expression of a gene.
 64. The method as claimed in claim 59, wherein said composition is injected for the detection, localization and imaging of the stem cells of the brain, and wherein said nucleic acid encodes a reporter protein or constitutes a signal-generating marker.
 65. The method as claimed in claim 64, wherein said reporter protein is luciferase, green fluorescence protein, β-galactodidase or chloramphenicol acetyltransferase.
 66. An animal, with the exception of a human, comprising at least one stem cell of a brain which has undergone at least one event of site-specific recombination targeted to a selected DNA sequence, wherein said animal is produced using a method which comprises the steps of: a) obtaining a totipotent embryonic stem cell modified by insertion of a recognition site for a site-specific recombinase protein into said DNA sequence, located in one or more chromosomes, by homologous recombination; b) introducing said modified totipotent embryonic stem cell into an embryo of an organism of said animal; c) selecting said animal having a genetic modification integrated into germinal cells; d) developing said animal; e) introducing at least one nucleic acid/cationic polymer complex composition into the brain of said animal obtained in step (d) close to or into ventricular stem cells of said brain, said nucleic acid at least encoding a recombinase protein capable of catalyzing the recombination between said recognition sequences for said recombinase; and f) expressing said recombinase protein in said cell.
 67. The animal as claimed in claim 66, wherein at least a second nucleic acid/cationic polymer complex composition is introduced into the brain of said animal in step (e) of the method of production, said second nucleic acid encoding a protein product which is a cytokine, lymphokine, interleukine, transcription factor, survival factor, anti-apoptotic protein, pro-apoptotic protein, growth factor, recombinase, integrase, transposase, enzyme involved with DNA, differentiation factor or reporter protein.
 68. An animal intended for the screening of compounds intended to modify the evolution of stem cells of the brain of the animal, comprising producing said animal using a method which comprises the following steps: (a) preparing a nucleic acid/cationic polymer complex composition; (b) introducing said composition into the brain of said animal close to or into the ventricular stem cells of the brain; and (c) allowing said nucleic acid to penetrate into said stem cells.
 69. The animal as claimed in claim 68, wherein said composition is introduced by intraventricular stereotactic injection.
 70. The animal as claimed in claim 68, wherein said nucleic acid is double-stranded DNA which is integrated into a genome of the stem cells of the brain.
 71. The animal as claimed in claim 70, wherein said double-stranded DNA is a vector which is transposon, retroviral vector or an adeno-associated virus-derived vector.
 72. The animal as claimed in claim 68, wherein said nucleic acid is a double-stranded nucleic acid which is present in an episomal state.
 73. The animal as claimed in claim 71, wherein said vector is self-replicating.
 74. The animal as claimed in claim 66, wherein said nucleic acid encodes a protein product the expression of which is placed under the control of a promoter or of a promoter region allowing expression of a transgene constitutively or in a tissue-specific or cell type-specific manner.
 75. The animal as claimed in claim 74, wherein said tissue-specific or cell type-specific promoter is an oligodendrocyte-specific promoter, a neuron-specific promoter, or an astrocyte-specific promoter.
 76. A method for a screening compound for modifying the evolution of stem cells of a brain, comprising the steps of: a) bringing an animal as claimed in claim 68 into contact with said compound to be screened; and b) analyzing the evolution of the stem cells of the brain of said animal.
 77. A compound obtained using the method claimed in claim
 76. 78. The compound as claimed in claim 77, wherein the compound is useful for preparing a composition for the treatment of neurodegenerative and/or demyelinating diseases. 